In a cellular communication system, each of the subscriber units (typically mobile stations, communication terminals, wireless devices, user equipment, remote terminals etc) communicates with a fixed base station. Communication from the subscriber unit to the base station is known as uplink, and communication from the base station to the subscriber unit is known as downlink. The total coverage area of the system is divided into a number of separate areas or cells, each predominantly covered by a single base station. The cells are typically geographically distinct with an overlapping coverage area with neighbouring cells. FIG. 1 illustrates a cellular communication system 100. In the system, a base station 101 communicates with a number of subscriber units 103 over radio channels 105. In the cellular system, the base station 101 covers users within a certain geographical area 107, whereas other geographical areas 109, 111 are covered by other base stations 113, 115. Some overlap areas 117 can be covered by more than one cell.
As a subscriber unit moves from the coverage area of one cell to the coverage area of another cell, the communication link will change from being between the subscriber unit and the base station of the first cell, to being between the subscriber unit and the base station of the second cell. This is known as a handover. Specifically, some cells may lie completely within the coverage of other larger cells.
All base stations are interconnected by a fixed network. This fixed network comprises communication lines, switches, interfaces to other communication networks and various controllers required for operating the network. A call from a subscriber unit is routed through the fixed network to the destination specific for this call. If the call is between two subscriber units of the same communication system, the call will be routed through the fixed network to the base station of the cell in which the other subscriber unit currently is. A connection is thus established between the two serving cells through the fixed network. Alternatively, if the call is between a subscriber unit and a telephone connected to the Public Switched Telephone Network (PSTN) the call is routed from the serving base station to the interface between the cellular mobile communication system and the PSTN. It is then routed from the interface to the telephone by the PSTN.
Traditional traffic in mobile cellular communication systems has been circuit switched voice data where a permanent link is set up between the communicating parties. In the future, it is envisaged that data communication will increase substantially and typically the requirements for a remote terminal to transmit data will not be continuous, but will be at irregular intervals. Consequently it is inefficient to have a continuous link set up between users. Instead a significant increase in packet based data traffic is expected, where the transmitting remote terminal seeks to transmit the data in discrete data packets when necessary. An example of a packet based system is the General Packet Radio Service (GPRS) introduced for the Global System for Mobile communication (GSM). Further details on data packet systems can be found in ‘Understanding data communications: from fundamentals to networking, 2nd ed.’, John Wiley publishers, author Gilbert Held, 1997, ISBN 0-471-96820-X.
A cellular mobile communication system is allocated a frequency spectrum for the radio communication between the subscriber units and the base stations. This spectrum must be shared between all subscriber units simultaneously using the system.
One method of sharing this spectrum is by a technique known as Time Division Multiple Access (TDMA). In a TDMA communication system, the frequency spectrum is typically divided into a number of separate frequency channels or carriers and for each of these carriers a plurality of subscriber units are served by being allocated distinct time intervals. Thus in the example of the IDMA communication system GSM, the frequency spectrum is divided into 200 kHz frequency channels each of which is divided into eight separate time slots. A subscriber unit is allocated a specific time slot on a given frequency channel for communication with the serving base station. Further details of GSM can be found in ‘The GSM System for Mobile Communications’, Bay Foreign Language Books, authors Michel Mouly and Marie-Bernadette Pautet, 1992, ISBN 2950719007.
Another method of sharing cellular spectrum is by a technique known as Code Division Multiple Access (CDMA). In a Direct Sequence CDMA (DS-CDMA) communication system, the signals are multiplied by a high rate code whereby the signal is spread over a larger frequency spectrum. A narrowband signal is thus spread and transmitted as a wideband signal. At the receiver, the original narrowband signal is regenerated by multiplication of the received signal with the same code used to spread the signal in the transmitter. A signal which has been spread by use of a different code will not be de-spread by the receiver but will remain a wide band signal. It will then be removed by filtering after the de-spreading operation. In the receiver, the majority of interference from signals in the same frequency spectrum as the wanted signal can thus be removed by filtering. Consequently, a plurality of subscriber units can be accommodated in the same wideband spectrum by allocating different codes for different subscriber units. Codes are chosen to minimise the interference caused between subscriber units typically by choosing orthogonal codes when possible. A further description of CDMA communication systems can be found in ‘Spread Spectrum CDMA Systems for Wireless Communications’, Glisic & Vucetic, Artech house Publishers, 1997, ISBN 0-89006-858-5. Examples of CDMA cellular communication systems are IS 95 and the Universal Mobile Telecommunication System (UMTS).
Currently many cellular communication system operators are in the process of rolling out UMTS or other 3G communication systems. It is typically intended that these 3G systems will interoperate with the 2G communication systems such as GSM. Specifically, it is expected that UMTS communication systems will initially be rolled out in separate islands of coverage, and consequently most of the initial release mobiles will be dual mode UMTS/GSM mobiles, in order to enable use outside areas of UMTS coverage. Effective handover between UMTS and GSM is thus imperative when mobiles roam in and out of UMTS coverage.
UMTS and GSM standards are being written in a way to enable implementation of two types of multi-RAT (Radio Access Technology) mobiles, namely single receiver and dual receiver mobiles. Dual receiver mobiles can simultaneously receive both GSM and UMTS signals, so these mobiles can quickly perform measurements necessary for inter-system handover during active calls. However, single receiver mobiles can only measure other systems during idle periods, when the receiver of the subscriber unit is not active in receiving a call. In order to permit single receiver subscriber units to measure during an active UMTS call a compressed mode of operation has been standardised. In this method, the information of the active call is transmitted at increased data rate, thereby freeing up a time interval during which GSM measurements can be made. Single receiver mobiles in active GSM calls can perform UMTS measurements during GSM idle frames (the mobile synthesizer switching time does not allow UMTS measurements during idle GSM time slots). This results in long inter-RAT synchronisation times, e.g. a single receiver mobile in an active GSM call may require 1 second to synchronise to one UMTS cell under good channel conditions and 5–16 seconds under poor channel conditions. This is a highly unacceptable delay, which will result in decreased handover performance and an increased number of dropped calls.
Hence, the current schemes for measuring carriers result in unacceptable delays and degraded handover performance and there is thus a need for an improved system for measuring carriers.